This invention relates, but is not necessarily limited to, closed loop hydraulic swing circuits for cranes or the like, and more particularly to solving a problem that can occur when the load is allowed to coast and results in an overrunning torque load that causes an excess flow condition in which the motor returns more fluid to the pump than the pump can accept.
A conventional hydraulic swing system includes a reversible variable displacement pump driven by an engine, and a bi-directional hydraulic motor connected in a closed loop with the pump. There are appropriate relief valves connected across the loop to relieve respective main lines when the pressure exceeds a predetermined maximum system pressure, the relief valves being piloted off their respective main lines. There is a primary control valve to selectively pressurize pump control lines, and a secondary control or null-out valve to provide smooth acceleration.
The operator of such a system will often allow the load to coast. This is done by increasing engine speed and stroking the pump for a brief period of time to build up momentum, and then allowing the engine to idle and returning the primary control valve to neutral. During coasting, the secondary control valve is responsive to pressure in what is then the return line and, if required, will cause the pump to stroke to full displacement in the direction required to accept fluid being returned by the motor. Particularly if, as often happens, the operator has initiated a high coasting speed, situations can arise in which the pump, even at full displacement, cannot accept all of the returning fluid and the result can be a sudden and/or erratic braking action that can cause serious damage to the machine, the load, or surrounding structures that the load may strike in a resulting swinging action. The associated system relief valve will ultimately relieve pressure in the return line, but the setting is necessarily too high to prevent braking of a magnitude sufficient to cause damage. The situation can also be corrected by accelerating the engine to increase pump speed, and therefore the ability of the pump to accept fluid, but it is not a natural act for the operator to accelerate when faced with an overrunning load and so it is not likely that the proper step will be taken before damage is done.